Magnetic Particle Imaging (MPI) is a promising imaging procedure for medical diagnosis purposes. In MPI, the local concentration of magnetic nano-scale particles is measured quickly and at high precision to calculate an image with high steric resolution. With use of biocompatible nanoparticles, MPI can be utilized for many applications in medicine, such as cardio-vascular diagnosis for arteriosclerotic plaques, or to monitor the blood supply of the heart muscle, or of certain brain areas or the extremities.
In the MPI technique, only the interaction of magnetic particles with an alternating magnetic field contributes to signal generation. To achieve sufficient signal intensity and thus a good image resolution, magnetic particles with a sufficient sensitivity to an alternating magnetic field should be used. Magnetic particles can be distinguished as so called Brown-magnetic particles, which align their steric orientation to an external magnetic field, and so called Neél-magnetic particles which do not align their steric orientation, but do align their magnetic spin to an external magnetic field.
If the external magnetic field is alternating in its direction, magnetic particles will follow the field change either by changing their steric or magnetic spin orientation. At high frequencies above 10 kHz of the alternating magnetic field, only the Neél-magnetic particles are able to follow the field change. Accordingly, only Neél-magnetic particles contribute to signalling in high resolution MPI and preferably magnetic particles with a high proportion of Neél particles in view of Brown particles are utilized in MPI.
It is known to subject magnetic particles to an alternating magnetic field for heating purposes (DE 19800294 A1, US2003/0211045 A1), analysing biochemical molecules on a substrate (DE 10 2006037739A1, EP0926496 B1) or separating magnetic particles from a suspension (US 2009/0151176 A1) all incorporated herein by reference in their entireties. For the teachings of these publications, the differences between Neél- and Brown magnetic particles is not of concern. Accordingly, separation of Brown- and Neél-magnetic particles is not disclosed.
The use of Neél-magnetic particles for MPI and a procedure for the enrichment of Neél-magnetic particles is known and described, for example, in patent publication WO 01/10558 A1, also incorporated by reference in its entirety. This publication discloses a device for the separation of Neél- and Brown-magnetic particles using an alternating magnetic field. The alternating magnetic field operates at a frequency of 1 mHz to 100 GHz and the magnetic particles have a size of 0.1 nm to 100 μm. To enhance the alternating magnetic field, WO 01/10558 A1 proposes introducing spherical separation particles into the magnetic field. However, spherical separation particles result in such a high magnetic gradient that both Neél- and Brown-magnetic particles are immobilized by the alternating magnetic field and the separation of the particles is not satisfactory. Furthermore, the particles immobilized at the proposed separation particles are difficult to be removed from separation particles, which is attributed to the material used in WO 01/10558 A1.